In the medical study of respiration, including monitoring during sleep, it often is important to detect and classify respiration using the least invasive method possible. To this end the measurement of airflow in the proximity of the nose and mouth (i.e. oro-nasally) is an established technique. Although the majority of people breath through their nose while asleep it is important also to monitor oral airflow. The failure also to monitor oral airflow may lead to misinterpretation of mouth breathing as a cessation (apnea) or reduction (hypopnea) in respiration.
One of two methods of measurement are conventionally used to monitor oro-nasal airflow. The first is based on the location of a thermistor (or thermistors) in the oral and nasal airflows, as shown in FIG. 1. The thermistors 1 are connected to electronic circuitry 2 which measures their electrical resistance and outputs a signal 3 indicative of this resistance and/or a change therein. Airflow past the thermistor is measured by the increase in temperature of the exhaled air relative to ambient, as shown in FIG. 2, or alternatively by the cooling effect of a moving airflow past a thermistor which is warmed above ambient by an electrical current passing through it.
While this method of measurement is convenient and cheap, the low frequency cut-off point of the filtering circuitry necessary to remove noise from the flow signal to make it useable also removes higher frequency elements indicative of snoring or respiratory flow limitation. The phase response of such filtering also may distort the timing of respiration.
A second known method of measuring oro-nasal airflow, shown in FIG. 3, uses the well known pitot tube or Bernoulli effect, whereby a pressure which varies with flow rate is generated in a tube 4 by placing its open end parallel with, or at some intermediate angle to, the flow. The other end of the tube terminates at an electrical pressure transducer 5, the output signal 6 of which thus varies with flow rate.
Unlike the thermistor technique, the flow measurements derived from the pressure transducers 5 have a bandwidth adequate for detecting both snoring and flow limitation. FIG. 4 shows a typical pressure signal illustrating both instances. Such systems are routinely used for the detection of nasal respiration, but their use in measuring oral respiration is problematic due to the lower flow velocities often found in the wider oral cross-section compared to the narrower nasal passageway.
When the pressure transducer prior art method is used to measure both nasal and oral airflow, normally only one pressure transducer is fed by two sources. Two tubes 7 are located at the periphery of or just inside, the nares, and another single (or double) tube 8 is located in the vicinity of the lips, as shown in FIG. 5. These tubes join downstream into a common tube 9 in communication with the circuitry 5.
Oral flow measured in this way gives a significantly lower output (typically a factor of 6) from the pressure transducer than nasal flow. Additionally, the oro-nasal tube configuration further attenuates the oral flow by a factor of about two because for zero nasal flow there is a pressure drop down the tubing path from the oral inlet 8 to the nasal inlets 7, as exemplified in FIG. 6. This pneumatic "potential divider" effect is present in all configurations where two tube inlets join together via similar tubes.
The sensitivity of the oral channel also is highly dependent on the positioning of the oral tube 8. As shown in FIGS. 7a and 7b, if the tube end 10 is located in the centre of the airflow for exhalation with slightly opened lips 11 it will become insensitive if the mouth is opened further and the airflow profile changes.